1. Field of the Invention
This invention relates to cathode ray tubes, and in particular, to circuits for reducing pincushion distortion.
2. Description of the Related Art
Cathode ray tubes (CRTs) typically display video images by deflecting an electron beam with a yoke having horizontal and vertical deflection coils. The electron beam is swept across the CRT face or screen to display horizontal scan lines on the face using a horizontal deflection current, typically having a sawtooth waveform, applied to the horizontal deflection yoke coil of the CRT. The CRT face is roughly planar and rectangular, rather than spherical. Accordingly, different deflection angle ranges are required to sweep a scan line across the entire width of a CRT screen at different vertical scan line locations.
Using such non-spherical CRT faces thus gives rise to display artifacts known as pincushion distortion. Referring now to FIG. 1, there is shown a CRT screen 100 illustrating such pincushion distortion. At a constant peak-to-peak magnitude of the horizontal deflection current that causes the electron beam to sweep across the CRT screen, scan lines 121 and 123, near the top and bottom edges of screen 100, respectively, have a different length than scan lines closer to the vertical center, such as center scan line 122, in part because of the different deflection angle ranges required for different scan lines. This causes a distortion such that a rectangle to be displayed on screen 100 would have its vertical edges squeezed inwards, so that they were narrowest near the vertical center of screen 100, as illustrated by curves 112.
Such pincushion distortion is typically addressed by a pincushion modulator circuit that modulates the deflection current driving the horizontal deflection yoke coil so that it is decreased as scan lines are farther from vertical center, and increased for scan lines closer to vertical center. The pincushion modulator circuit, sometimes called a pin modulator circuit or diode modulator circuit, typically modulates an input sawtooth current in accordance with a modulation voltage having a parabolic-shaped waveform provided by a pincushion control circuit. Pincushion distortion, pincushion modulator circuits, and related aspects of CRT systems and pincushion correction are described in K. Blair Benson, Television Engineering Handbook (New York: McGraw Hill Book Co., 1986), at pages 13.175 to 13.181.
Different CRTs can require deflection currents of different magnitude ranges to drive the horizontal deflection yoke coil. Different deflection current magnitude ranges can require different modulation voltage ranges. For a given pincushion control circuit, a change in the CRTs can thus require modulation voltages having a parabolic waveform with accordingly different peak-to-peak magnitudes generated by the circuit. For example, for a given pincushion control circuit, changing the faceplate geometry of the CRT can require larger deflection currents, which thus requires modulation voltages having a parabolic waveform with larger peak-to-peak magnitudes. One problem with such pincushion distortion correction systems is that parabolic voltage waveforms may require magnitudes larger than the maximum output modulation voltage of the control circuit. For example, if an internally compensated operational amplifier is utilized as part of the output stage of the control circuit, its output voltage will be limited to the supply voltage of the operational amplifier (sometimes referred to as the supply voltage rails of the operational amplifier). This may require changing the circuitry, for example to utilize an operational amplifier having a larger supply voltage.